Struct x86_64::structures::gdt::GlobalDescriptorTable

pub struct GlobalDescriptorTable { /* fields omitted */ }

A 64-bit mode global descriptor table (GDT).

In 64-bit mode, segmentation is not supported. The GDT is used nonetheless, for example for switching between user and kernel mode or for loading a TSS.

The GDT has a fixed size of 8 entries, trying to add more entries will panic.

You do not need to add a null segment descriptor yourself - this is already done internally.

Data segment registers in ring 0 can be loaded with the null segment selector. When running in ring 3, the ss register must point to a valid data segment which can be obtained through the Descriptor::user_data_segment() function. Code segments must be valid and non-null at all times and can be obtained through the Descriptor::kernel_code_segment() and Descriptor::user_code_segment() in rings 0 and 3 respectively.

For more info, see: x86 Instruction Reference for mov, Intel Manual, AMD Manual

Example

use x86_64::structures::gdt::{GlobalDescriptorTable, Descriptor};

let mut gdt = GlobalDescriptorTable::new();
gdt.add_entry(Descriptor::kernel_code_segment());
gdt.add_entry(Descriptor::user_code_segment());
gdt.add_entry(Descriptor::user_data_segment());

// Add entry for TSS, call gdt.load() then update segment registers

Implementations

impl GlobalDescriptorTable

pub const fn new() -> GlobalDescriptorTable

Creates an empty GDT.

pub const unsafe fn from_raw_slice(slice: &[u64]) -> GlobalDescriptorTable

Forms a GDT from a slice of u64.

Safety

• The user must make sure that the entries are well formed
• The provided slice must not be larger than 8 items (only up to the first 8 will be observed.)

pub fn as_raw_slice(&self) -> &[u64]

Get a reference to the internal table.

The resulting slice may contain system descriptors, which span two u64s.

pub const fn add_entry(&mut self, entry: Descriptor) -> SegmentSelector

Adds the given segment descriptor to the GDT, returning the segment selector.

Panics if the GDT has no free entries left. Without the const_fn feature, the panic message will be “index out of bounds”.

pub fn load(&'static self)

Loads the GDT in the CPU using the lgdt instruction. This does not alter any of the segment registers; you must (re)load them yourself using the appropriate functions: SS::set_reg() and CS::set_reg().

pub unsafe fn load_unsafe(&self)

Loads the GDT in the CPU using the lgdt instruction. This does not alter any of the segment registers; you must (re)load them yourself using the appropriate functions: SS::set_reg() and CS::set_reg().

Safety

Unlike load this function will not impose a static lifetime constraint this means its up to the user to ensure that there will be no modifications after loading and that the GDT will live for as long as it’s loaded.

Trait Implementations

impl Clone for GlobalDescriptorTable

fn clone(&self) -> GlobalDescriptorTable

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for GlobalDescriptorTable

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.